Public health experts are concerned about antibiotic overuse, and a new report from consumer groups and environmentalists looked at the practices of the top 25 burger chains. They gave 22 restaurants, including Burger King and McDonalds an “F.”

Wendy’s got a “D-” because it buys some beef from a supplier that uses reduced antibiotics.

“The problem of antibiotic resistance, which comes from overuse of antibiotics, both in humans and animals, is a crisis at this point,” said Jean Halloran, director of food policy initiatives for Consumer Reports.

The authors of report see the widespread use of antibiotics in meat production leading to the spread of drug resistant superbugs. The chicken industry recently came under fire for antibiotic use, prompting pledges from big chains, including McDonalds, Panera and Chipotle, to use chicken raised without antibiotics.

McDonalds said it’s working to roll out an antibiotics policy for beef by the end of the year.